Control system for electric furnaces



E A. HESTER.

CONTROL SYSTEM FOR ELECTRIC FURNACES.

APPUCATION FILED OCT. 30. 1918.

1,349,379. Patented Aug. 10, 1920.

\ f/ecir/c Furnace PM E W &

WiTRESSES: INVENTOR .fayar 4 Heifer ATTORNEY EDGAR A. nns'rnn, or EDo'EwooD PARK, PENNSYLVANIALASSIGNOR ro WESTING- HOUSE' ELECTRIC & MANUFACTURING COMPANY,

VANIA.

Application To all whom it may concern: 7 Be it known that I, EDGAR A. HESTER, a

' citizenof the United States, and a resident ghe'ny and of Edgewood Park, in the county of Alle- State of Pennsylvania, have invented a new and useful Improvement in Control Systems for Electric Furnaces, of which the following is a specification.

My invention relates to control. systems for electric furnaces and it has special rela 'tion' to those electric furnaces in which the resistance is of starting temperature comparatively high at the time and which possess a negative coefficient during the operation thereof.

Since the current in a furnace of the char acter above designated increases as the temperature of the furnace increases, 1t 18 necessary to associate with a furnace of such character a control system which will operate to decrease the voltage impressed upon the furnace as the temperature rises and, when the proper temperature has been reached, to so maintain the desired voltage that the conditions within the furnace will become stabilized. It has heretofore been suggested that the desired control could be obtained by associating thermal relays with furnaces of this. type. However, it is not possible, from an operating standpoint, to directly connect the switches necessary to vary thepower supplied to a furnace with a thermal relay in such a manner that satisfactory conditions will be attained. This follows from the fact that the thermal relay, by reason of its delicate construction, can not carry sufficient current to operate the magnetic coils usually employed with switches of this type.

It is, therefore, necessary to interpose, be-

tween the main switching mechanism. and

the relay which controls this mechanism, such means as will transfer the energy from the switching means, and it is with this transfer apparatus that difficulty has been met. The apparatus in question. must be of a compact and simple nature and not be liable to get out of order under the extremely severe operating conditions which pertain to electric furnaces of the type above-mentioned. Moreover, it is also necessary that such transfer means be reversible in character, that is, not only must the proper control he maintained when the tem- Specification of Letters Patent.

signed to ,A CORPORATION OF PENNSYL- CONTROL SYSTEM FOR ELECTRIC FURNACES.

Patented Aug. 10, 1920.

filed October a0,- 1918. Serial No. 2eo,272.

perature in the furnace is increasing but the control mechanism must also operate in such a manner that, should the furnace tend to become excessively hot, it couldbe removed from its source of power supply and, therefore, be allowed to cool for a predetermined time.

The objects of my invention are, therefore, to obtain the desired control of an electric furnace and to maintain the temperature in such furnace, in themanner above described, with a minimum amount of switching apparatus and with standard control apparatus, so far as possible.

For a better understanding of my invention, reference may be had to the accom- 'panying drawing, the single figure of which illustrates diagrammatically a control sys tem constructed I in accordance with my invention. A furnace l is shown as supplied with power from the secondary-2 of a transformer 3. A primary winding 4 of the trans former 3 derives energy from a supply line 5. A: thermal relay (3 is shown as operatively associated with the furnace 1 through a current transformer 7 which is inductivel interlinked with one of the leads of the furnace. The energy induced in the secondary winding of the transformer 7 is translated into heat in a heating element which is disposed within the thermal relay 6.

Contact members 9, 10, 11, and 12 are disposed peripherally upon the relay 6. Associated with the respective members'9, 10, 11 and 12 are pairs'of terminals 1314, 15-16, 1718, and 1920, the contact members 9, 10, 11 and 12 oscillating between their associated pairs of terminals. purpose 'of convenience, only three of the contact members, namely,men1'bers 9, 10,

and 11 are 'used for control in the present inassociated pairs of terminals may be used and, 'in this manner, control obtained.

For a better understanding of the thermal relay as shown, reference maybe had to a copending application, Serial No. 269,093, filed Dec. 31, 1918, by O. A. Colby, and as-.

the Westinghouse any desired range of om pany. Magnetically operated. switches 91, 22 and Elec. & Mfg.

For

Eli!

' double-pole,

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tilt

the terminal 14.

23 serve to vary the number of effective turns of the primary transformer 3 and, by their successive closure therefore, also vary the amount of voltage impressed upon the furnace 1. i 4

Transfer switches 24, 25 and 26 are interposed between the relay 6 and the switches 21, 22 and 23 and are adapted to beactuated by energy derived from the line Actuating vcoi ls 27 and 28 are associated with the and 30 with the switch 32 .with the switch 26. Furthermore, the movable member 83 of the switch 24 is adapted to be moved between terminals 36 and 37; the member 34 of switch 25 between terminals 38 and 39; and the member of the switch 26 between terminals 40 and tl.

A. regulating resistance 42 is shown as connected in the relay circuit and is used for purposes hereinafter to be described. The secondary winding 2 of the transformer 3 is divided into two portions, adapted for either series orparallel connection by a switch as.

A line switch 44;.is provided to energize the conductors 5 ,fi'om a source of electrical energy (not shown).

Having described a system of connections as used with my improved system of control, the operation thereof is as follows: Assume that the cold stand and that, therefore, its resistance is of maximum value. Also assume that the relay 6 has its contact member 9 engaging Obviously, any of the associated contact switch 24, coils 29 25, and coils 31 and other terminals and their members could beassumed as closed and the.

operation of the system varied in accordance therewith but, for the purposes of illustration, I have shown that pair of terminals which are placed closer together as the ones to be first engaged.

The line switch 44 is closed and the magnetically operated switch 21 is. therefore, also closed, and the highest possible voltage is impressed upon the furnace. At this point in the operation of the furnace, the double-throw switch 43 is usually thrown into such position, that the two portions of the transformer secondary are in series relation. Vt'hen working conditions have been reached, these portions of the transformer secondary are connected in parallel.

As the furnace heats up, the resistance gradually decreases and an increased current tends to flow in the current leads. This results in the thermal relay 6 rotating in a clockwise direction. Upon rotation in this direction, the member 9 engages the terminal 13. whereupon a circuitis ompletcd through the coil 22% and the associated switch arm 33 is pulled into the position shown by the dotted lines, that is, the switch arm is member 45 of the.

which are furnace is started from a' wise tion indicated .will bring the contact men ber 10 into engagement with the terminal 16 whereupon the coil 30 will-be energized and the arm 3i moved into contact with the terminal 39. This action will establish a closed-circuit which will allow the switch 23 to be closed while the disengagement of the lever 3% from the terminal 38 will result in the opening of switchQZ. This last switching operation will result in the lowest possible voltage being impressed upon the furnace. v

Should the current now further increase because of an exceptionally high temperature existing in the furnace, the member 11 would make contact with the terminal 18, whereupon, the coil 32 would be energized and the switch arm 35 be moved to engage theeterminal 41. By such operation, the switch 23 would be opened and, conse' qucntly, no voltage would be impressed upon the furnace which would, therefore, start to cool.

The cooling operation will continue until the relay moves sufficiently in a counterclockwise direction to cause the contact member 9 to engage-the terminal 1t. hen this connection takes place, the coil 27 is energized, and the arm 33 moves back into its original position. The same sequence of events as above described then proceeds and, ultimately, the furnace temperature becomes stabilized. It will be noted that a considerable time interval will elapse before the relaymoves sufficiently in a counter-clockdirection to make the connectiens the furnace is again supplied with This is a feature of considerable inasmuch as the furnace has time high resistance to of current therewhereby power.

advantage, to cool sufficiently to a be offered to the flow through. In other words, a thermal relay automatically sets its own time limit and is particularly adapted for this type of apparatus. Moreover, as will be noted in following out the various steps, the transfer switches operate in such manner that. at no point, do the relay contact members break a circuit. That is, while the contact members always make a connection and thereafter cause current to flow, when the time arrives for that connection to be broken, the controller switches have 0 erated in such a mannenthat there is no longer current passing at that point. The

resistor 42 may be used to vary the amount of the current flowing through the heating coil of the thermal relay 6. If the resistor 42 is so adjusted as to carry a larger amount of current than before, the current value in the furnace will be larger before the thermal relay will operate as described above, and,

therefore. the furnace will be maintained at a higher temperature. I

While I have described but one embodiment of my invention, it is obvious that many modifications may occur to those skilled in the art and, therefore, I wish only those restrictions to be placed upon my .invention as are imposed by the prior art or are specifically set forth in the appended claims.

I claim as my invention:

1. The combination with an electric furnace and a sourcev of power supply therefor of a thermal relay operatively so associatei therewith that a movement proportional to the change of current in said furnace is ob- 'taken by said furnace,

tained, and switching means requiring an actuating current larger than can be used with said relay, for varying the amount of power. taken by said furnace.

' The combination with an electric furnace and-a source of power supply therefor, of a thermal relay operatively associated therewith. switching means requiring an actuating current larger than can beuscd with said relay. for varying the amount of power and transfer means interposed between said relay and said switching means whereby they may be operatively connected.

The combination with an electric furnace and a source of power supply therefor, of thermally responsive means operatively so associated therewith that a movement proportional to the change of current in said furnace is obtained. and switching means connected to said relay to "tll'y the power input of said furnace in accordance with the current therein.

4. The combination with an electric fur nace and a source of power supply therefor, of a thermally responsive relay operatively so associated therewith that a movement proportional to the change of current in said furnace is obtained, said relay having con tact members being adapted to oscillate between two terminal points associated there.

upon, switching means connectedto said resupply, a thermal relay lay, terminals operatively associated with each of said relay contact members, and leads connecting said switching means to said terminals whereby the movement of said thermal relay controls the movement of said switches.

6. The combination with an electric furnace and a source of power supply therefor, of a thermally responsive relay operatively so associated therewith that a movement proportional to the change of current in said furnace isobtained, said relay having contact members peripherally disposed thereupon, switching means connected to said relay, terminals operatively associated with each of said relay contact members, and leads connecting said switching'means to said terminals in such a manner that said relay does not. at any time, break the current at said terminals. whereby the movement of said thermal relay controls the movement of said switches.

7. The combination with an electric furnace and a source of power supply therefor, of switching means for varying the amount of power supplied to said furnace. a thermal relay operatively associated with said furnace. said relay being provided with a plurality of peripherally disposed contact members oscillating between terminals, each coin tact member being associated with a pair of terminals. and means connecting said terminals and said switching means.

8. The combination with an electric fu r nace and a source of power supply therefor, of switching means controlling said power supply. a thermal relay actuated in accordance with the current in said furnace, and transfer means connected between said switching members and said relay, said transfer means comprising a switching member adapted to be actuated by two coils.

9. The combination with an electric furnace and a source of power supply therefor, of switching means controlling said power supply, a thermal relay actuated in accordance with the current in said furnace, and transfer means connected between said switching and said relay, said transfer means comprising a switching member adapted to be actuated by two coils, one of said coils being energized when the current in said furnace is increasing and the other of said coils being energized when said current decreases.

10. The combination with an electric furnace and a source of power supply therefor, of switching means controlling said power actuated in accordance with the current in said furnace, and transfer means connected between said switching means and said relay. said transfer means comprising a switching member adapted to be actuated by two coils, one of said coils being energized when the current in said furnace is increasing and the other of said coils being energized when said current decreases.

11. The combination with an electric furnace and a source of power supply therefor, of switchingmeans controlling said power supply, a thermal relay comprising a heating element, means for supplyii i ig to said heating element a current proportional to, and derived from, the current flowing through said furnace, and transfer means connected between said switching members and said relay, said 'ransfer means comprising a switching member adapted to be actu ated by two coils.

12. .T he combination with an electric furnace and'a source of power supply therefor,

of switching means controlling said power supply. a thermal relay comprising a heat-- ing element, means for supplying to said heating element a current proportional to, and derived from, the current flowing through said furnace, transfer means connected between said switching members andsaid relay, said transfer means comprising a switching member adapted to be actuated by two coils, and means for varying the ratio of the two currents. I

13. The combination with an electric furnace and a source of power supply therefor,

of a thermally responsive relay operatively' so associated therewith that a movement proportional to the change of current in said furnace is obtained, said relay comprising so associated therewith proportional to the change of current in said contact members peripherally disposed thereon and a heating element, means for energizing said heating element by a current derived from, and proportional to, the current flowing through said furnace, switching means connected to said relay, terminals operatively associated with each of said relay contact members, and leads conecting said switching means to said terminals whereby the movement of said thermal relay controls the movement of said switches. let. The combination with an electric fun nace and a source of power supply therefor, of a thermally responsive'relay operatively that a movement furnace is obtained, said relay COHIPI1S1I1\ contact members peripherally disposed thereon and a heating element, means for energizing said heating element by a current derived from, and proportional to, the current flowing through said furnace, switching means connected to said relay, terminals operatively' associated with each of said relay contact members, leads connecting said switching means to said terminals whereby the movement controls the movement of said switches, and means for varying the ratio of the two currents. Y i a In. testimony whereof, I have hereunto subscribed my name this 26th day of Oct, 1918.

EDGAR A. ,HESTER.

so ofsaid thermal relay 

